The present invention relates to a power amplifier and, more particularly, to a power amplifier equipped with a differential amplifier input stage and a single-ended push-pull output stage.
In conventional power amplifiers, the input signal is received on the input stage and is voltage-amplified on the driver stage. It is then power-amplified on the output stage of a single-ended push-pull type to supply the power to the load. In these power amplifiers, the output is normally fed back to the input stage to form a negative feedback. In many instances, a differential amplifier is used as the input stage to increase the input impedance. A bias voltage is applied to a non-inverting input terminal of the differential amplifier in the input stage by resistance dividing of the power supply voltage. An input signal is also applied to the non-inverting input terminal. Appropriately attenuated output from the output stage is applied to the inverting input terminal as a negative feedback signal. In such a circuit configuration, the output mid-point potential is almost equal to the input bias voltage. For this reason, an input bias voltage is normally set at a voltage about half that of a power supply voltage.
When operating this power amplifier with a sufficiently large power supply voltage, the voltage at the output point of the output stage can swing to the full power supply voltage, thus deriving a large output with less distortion and increasing the utilization efficiency of the power supply. When the power supply voltage is low, deficiencies, such as noted below, result. The output stage is constructed by a series connected PNP and NPN transistors. When they are built in a semiconductor integrated circuit, it is necessary to form the output stage by a compound PNP and NPN transistor which combines two or more transistors and makes them an equivalent transistor. For this reason, the threshold voltages (the input voltages to start current flowing) of the compound PNP and NPN transistors differ, resulting in a deficiency that the output signal distorts at one half of the cycle in spite of no distortion at the other half when the power supply voltage becomes low. Thus, if the power supply voltage becomes low, the non-distorted output becomes still smaller in amplitude. This deficiency deteriorates the utilization efficiency of the power supply voltage. This deficiency is more conspicuous when a bootstrap is applied to improve the drive efficiency of the output stage.